It is known to reserve programming of memories in an insulated gate transistor type of integrated circuit until late in the process of making the integrated circuit. Such processes typically involve ion implantation through polycrystalline silicon gate electrodes, to selectively adjust threshold voltages of particular insulated gate transistors in a memory matrix. If the memory matrix is of depletion type transistors, the programming implant reduces threshold voltage of selected transistors to below a predetermined operating voltage for the matrix. One reduces insulated gate transistor threshold voltage by implanting the channel region with ions of a conductivity type the same as that of its source and drain. We refer to an implant for reducing threshold voltage as a depletion implant.
However, in most instances the memory matrix is of enhancement type transistors. In such instance one programs the matrix with an implant that selectively raises, not reduces, the threshold voltage of the selected transistors above a predetermined operating voltage for the matrix. One increases the threshold voltage of an insulated gate transistor by implanting its channel with ions of a conductivity type opposite to that of its source and drain. We refer to an implant that increases threshold voltage as an enhancement implant.
Late programming is described in U.S. Pat. No. 4,294,209 and 4,299,862 to Donnelly. Also, late programming processes are described in detail in the earlier-filed U.S. patent application Nos. 252,203 by Donnelly, and 268,086, 268,088, 268,089 and 268,090 by Dickman et al. All of the foregoing patents and patent applications involve using an implant to program a read-only memory (ROM) matrix of enhancement-type insulated gate transistors. We also recognize that it is known to use an enhancement implant to program peripheral circuitry at the same time a ROM is being programmed.
We have now found that if one matches the depletion and enhancement implants and if one coordinates these implants with heating of the integrated circuit after the enhancement implant, one can program in still another way. In fact, one can even program power supply paths to various portions of the integrated circuit at the same time the ROM is being programmed. It is performed late in the fabrication process, and no added process steps are required.